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          06_镜像加载原理
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        <h3 id="1-镜像的概念"><a href="#1-镜像的概念" class="headerlink" title="1. 镜像的概念"></a>1. 镜像的概念</h3><p>再来回顾一下什么是镜像。</p>
<p>镜像是一种轻量级、可执行的独立软件包，用来打包软件运行环境和基于运行环境开发的软件，它包含运行某个软件所需的所有内容，包括代码、运行时、库、环境变量和配置文件。</p>
<h3 id="2-加载原理"><a href="#2-加载原理" class="headerlink" title="2. 加载原理"></a>2. 加载原理</h3><p>Docker 的镜像实际上由一层一层的文件系统组成，这种层级的文件系统叫做 UnionFS。</p>


<span id="more"></span>

<p>最底层的 bootfs (boot file system) 主要包含 bootloader 和 kernel，bootloader 主要作用是引导加载 kernel。这一层与我们典型的 Linux&#x2F;Unix 系统是一样的，当boot加载完成之后整个内核就都在内存中了，此时内存的使用权已由 bootfs 转交给内核，此时系统也会卸载 bootfs。</p>
<p>rootfs (root file system) 在 bootfs 之上。包含的就是典型 Linux 系统中的 &#x2F;dev，&#x2F;proc，&#x2F;bin，&#x2F;etc 等标准目录和文件。rootfs 就是各种不同的操作系统发行版，比如 Ubuntu，Centos 等等。</p>
<p>image 镜像就运行在这个迷你的 Linux 操作系统上。当镜像发生内容修改或新增时，就会在当前镜像层之上，创建新的镜像层。</p>
<p>对于一个精简的操作系统，rootfs 可以很小，只需要包含最基本的命令，工具和程序库就可以了。因为底层直接用宿主机的 kernel，自己只需要提供 rootfs 就可以了。所以对于不同的 Linux 发行版, bootfs 基本是一致的, 只是 rootfs 会有差别, 因此不同的发行版可以公用 bootfs。</p>
<h3 id="3-分层理解"><a href="#3-分层理解" class="headerlink" title="3. 分层理解"></a>3. 分层理解</h3><p>在之前下载镜像的时候，如果注意观察下载日志，可以看到整个镜像不是一口气整体下载下来的：</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">[root@horsneliu ~]<span class="comment"># docker pull mysql:5.7</span></span><br><span class="line">5.7: Pulling from library/mysql <span class="comment"># 根据tag下载</span></span><br><span class="line">b4d181a07f80: Pull complete <span class="comment"># 分层下载 联合文件系统</span></span><br><span class="line">a462b60610f5: Pull complete</span><br><span class="line">578fafb77ab8: Pull complete</span><br><span class="line">524046006037: Pull complete</span><br><span class="line">d0cbe54c8855: Pull complete</span><br><span class="line">aa18e05cc46d: Pull complete</span><br><span class="line">fd6f649b1d0a: Pull complete</span><br><span class="line">8a2b858b000b: Pull complete</span><br><span class="line">322182b17422: Downloading [===============================&gt;                   ]  67.82MB/108.6MB</span><br><span class="line">070e28050a88: Download complete</span><br><span class="line">613bdfd8796e: Download complete</span><br></pre></td></tr></table></figure>

<p>分层最大的好处就是资源可以共享。多个镜像都从相同的 Base 镜像构建而来，这样宿主机只需要在磁盘上保留一份 Base 镜像，同时内存中也只需要加载一份 Base 镜像，它为所有的容器服务，而且镜像的每一层都可以被共享。</p>
<p>查看一个镜像的分层：</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">[root@horsneliu ~]<span class="comment"># docker image inspect mysql:5.7</span></span><br><span class="line">......</span><br><span class="line">        <span class="string">&quot;RootFS&quot;</span>: &#123;</span><br><span class="line">            <span class="string">&quot;Type&quot;</span>: <span class="string">&quot;layers&quot;</span>,</span><br><span class="line">            <span class="string">&quot;Layers&quot;</span>: [</span><br><span class="line">                <span class="string">&quot;sha256:764055ebc9a7a290b64d17cf9ea550f1099c202d83795aa967428ebdf335c9f7&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:71a14cc55692a4edac5da18be420acf085c681ec95f7af5ff64bb1c236440d59&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:50854886015e8e202f3197c6a95ae60632a432aba673188bb7c3f9ee80ed2f30&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:1952fb2b0eb4d355f05a815c6d804243f089e05c0b8f3b1c1eaf0a1109e8293a&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:893f6aea2ce23f813c44180ccb7a41e8a4d151cb02f7c365cc4e837c0872b211&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:b8d0aeaeeee8ed55cb6bfc7e061d0bb0491e91024790c45d0c56b07c29cea080&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:98cf3cd0d5c613794ec0d66cdbf47cf1179ccdc27c8002b7abcfaa182659c946&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:5aa8927648f33f1a16a5118e8e4355ef539e52b1633a9d92dcd9122dc23b65b2&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:950c854d7d7a7bfea7aa27e37aa4a04da9b478800d6300571140f4156e773e5f&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:539a2455e1fb931673e021a4e46485fb0ac409b5bfdd41f465f0f69eb5f4b72f&quot;</span>,</span><br><span class="line">                <span class="string">&quot;sha256:8e7032ef053c9fb808fd6d9330a416a9eb94e4987ad38429bbb05ea57558f372&quot;</span></span><br><span class="line">            ]</span><br><span class="line">        &#125;,</span><br><span class="line">        <span class="string">&quot;Metadata&quot;</span>: &#123;</span><br><span class="line">            <span class="string">&quot;LastTagTime&quot;</span>: <span class="string">&quot;0001-01-01T00:00:00Z&quot;</span></span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">]</span><br></pre></td></tr></table></figure>

<p>如果未来某一天，这个镜像发生了更新，就会有一个新的镜像层被添加在最上面，其他所有的镜像层都不会被改变。</p>


<p>Docker 镜像都是只读的（所以图上有个锁），当容器启动时，一个新的可写层会被加载到镜像的顶部。这一层就是通常说的容器层，容器层之下的都叫镜像层。</p>
<h3 id="4-提交镜像-commit"><a href="#4-提交镜像-commit" class="headerlink" title="4. 提交镜像 commit"></a>4. 提交镜像 commit</h3><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">docker commit -m <span class="string">&quot;描述信息&quot;</span> -a <span class="string">&quot;作者&quot;</span> 容器ID 容器名[:标签名]</span><br></pre></td></tr></table></figure>

<p>把刚才复制过文件的 tomcat 提交成自己的 tomcat：</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">[root@horsneliu ~]<span class="comment"># docker commit -m &quot;add default works to webapps/&quot; -a &quot;horsen&quot; my-tomcat tomcat-horsen:0.1</span></span><br><span class="line">sha256:bcc280e94e4aafbc2dfbdae70eebfe91e3ae647d0fec1307604e601c30bddc5b</span><br><span class="line">[root@horsneliu ~]<span class="comment"># docker images</span></span><br><span class="line">REPOSITORY            TAG       IMAGE ID       CREATED              SIZE</span><br><span class="line">tomcat-horsen         0.1       bcc280e94e4a   About a minute ago   673MB</span><br><span class="line">tomcat                latest    46cfbf1293b1   6 days ago           668MB</span><br></pre></td></tr></table></figure>

<p><em>注意：容器名不能有大写字母</em></p>
<p>提交之后就生成了一个新的镜像，新的和旧的都是可以使用的。利用 commit 可以保存一个镜像的当前状态，类似于虚拟机的快照。</p>

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